Do as the pictures show (page stretch, yes):
Go to the main screen:

Right-click your Blockland shortcut and choose "copy":

Right-click where you want the mission editor shortcut to be and choose "paste shortcut":

It should look like this:

Right-click your mission editor shortcut and choose "rename":

Name it whatever you want (I chose "Blockland Mission Editor") and then click somewhere else on the screen to name it:

Right-click the mission editor shortcut and click on properties:

Do what is show in numerical order. Make sure you add " -mod editor" without the quotes to the target:

Finally double click the icon or right-click it and choose "open":

Press F11 while on a singleplayer game to start it up. If you can't figure it out on your own from here then you don't need to be using it.